1. Field of the Invention
The present invention relates to a decoding method of turbo codes used in a digital communication system and more particularly to a decoding method of turbo codes and a device for the same, for activating related decoders in parallel, optimally weighting on the components of log-likelihood ratio generated from respective constituent decoders, and combining weighted log-likelihood ratios to obtain diversity gain, thereby reducing a bit error rate and decreasing the number of iterations.
2. Description of Related Art
Generally, as a channel error occurring in a digital communication system, there are noise and fading which is a phenomenon of signal distortion caused by random characteristics of transmission materials. To cope with this error factor and increase reliability, an error correction method is provided.
A representative commercialized method among the error correction methods used for digital communication systems is a method using a convolutional encoder and a Viterbi decoder. As users"" demand for multimedia information, such as video and data, increases recently, an error correction method accomplishing a much better bit error rate is required. Among various techniques for enhancing error correction capability, a turbo code has been provided as a novel technique and now is in the center of attention.
According to the turbo code system, data sequence and interleaved data sequence are respectively encoded for transmission, and reliability obtained through respective constituent decoders is communicated during the decoding, thereby enhancing reliability.
With reference to the accompanying drawings, transmitter and receiver of turbo code as described above will now be reviewed. FIGS. 1a and 1b are block diagrams for respectively showing conventional turbo encoder and decoder for code rate 1/3.
As shown in FIG. 1a, the encoder comprises: interleaver 10 for scrambling the data sequence (d) according to a prescribed rule to provide outputs; first convolutional encoder 11 for convolutionally encoding the data sequence (d) to provide outputs; second convolutional encoder 12 for convolutionally encoding the interleaved sequence of the data sequence (d) to provide outputs.
In FIG. 1a, a reference character, x0, is the same as the data sequence (d), x1 is the sequence that the data sequence (d) has been encoded and x2 is the sequence that the data sequence (d) has been interleaved and then encoded.
With reference to FIG. 1b, the structure of the receiving party corresponding to the transmitting party having such structure as described above will now be described. The sequences y0, y1 and y2 are the decoder input sequences corresponding to the sequences x0, x1 and x2 respectively. The decoder comprises: first constituent decoder 21 for receiving and decoding based on the sequences y0, y1 and the a priori information from the second constituent decoder 22 to output a log-likelihood ratio and provide extrinsic information to the second constituent decoder 22; interleaver 20 for receiving and scrambling the sequence y0 and the extrinsic information from the first constituent decoder 21 according to the rule as in the interleaver 10 to provide outputs; second constituent decoder 22 for receiving and decoding based on the interleaved sequence of y0, the sequence y2 and the a priori information from the first constituent decoder 21 to provide outputs; and deinterleaver 23 for receiving data outputs from the second constituent decoder 22, recovering the original order of sequences to output a log-likelihood ratio and provide ethnic information to the first constituent decoder 21 as a priori information.
Before undertaking description on the operation of the conventional turbo decoder having such structure, it may be advantageous to set forth definitions of certain terms for the purpose of helping to understand the decoding of turbo code. The terms to be defined are xe2x80x9clog-likelihood ratioxe2x80x9d and components of the log-likelihood ratio, xe2x80x9cextrinsic informationxe2x80x9d, xe2x80x9ca priori informationxe2x80x9d, and xe2x80x9cchannel valuexe2x80x9d.
The log-likelihood ratio, (dk) means the log value of the ratio of the a posteriori probabilities of the dk. It can be estimated that dk is xe2x80x9c1xe2x80x9d if the log-likelihood ratio is a positive value and dk is xe2x80x9c0xe2x80x9d if the log-likelihood ratio is a negative value. As the magnitude of the log-likelihood ratio gets larger, the reliability of this estimation gets higher. The following formula is the definition of 1(dk), the log-likelihood ratio of the first constituent decoder 21.
[Formula 1]
wherein, xe2x80x9cobservationsxe2x80x9d are the input sequences of the first constituent decoder 21, namely, the sequences y0, Y1 and the a priori information from tile second constituent decoder 22. (Similarly, 2(dk) is defined as the log-likelihood ratio of the second constituent decoder 22 where xe2x80x9cobservationsxe2x80x9d are the input sequences of the second constituent decoder 22, namely, the interleaved sequences of y0, the sequence y2 and the a priori information from the first constituent decoder 21.) Such log-likelihood ratio can be divided into the following three components:
[Formula 2]
wherein, the second term in the right side is the a priori information from the second constituent decoder 22, the third term is the channel value of the dk and the first term is the extrinsic information obtained from the present constituent decoding and provided to the second constituent decoder 22 as a priori information for the next constituent decoding.
Therefore, a priori information means the data transferred from the previous constituent decoding and used for obtaining the log-likelihood ratio of the present constituent decoding, and extrinsic information means the data derived from the log-likelihood ratio obtained by the present constituent decoding and used as a priori information for the next constituent decoding.
Based upon the terms defined above, the conventional decoding method will now be described. Primarily, the first constituent decoder 21 computes a log-likelihood ratio using the sequences y0 and y1, and provides the extrinsic information that is one of components of the computed log-likelihood ratio to the second constituent decoder 22 as a priori information. The second constituent decoder 22 computes a log-likelihood ratio using the interleaved sequence of y0, the sequence Y2 and the a priori information from the first constituent decoder 21, and provides the extrinsic information that is one of components of the computed log-likelihood ratio to the first constituent decoder 21 as a priori information.
According to the system described above, the sequential path of first and second constituent decodings is defined as 1 iteration. After the time when the number of iterations, p, exceeds 1, the first constituent decoder 21 uses the extrinsic information from the second constituent decoder 22 as a priori information when computing the log-likelihood ratio.
After the iterations are performed a predetermined number of times, the hard decision is made on the latest log-likelihood ratio against a threshold xe2x80x9c0xe2x80x9d to estimate the dk. As the number of iterations increases, the bit error rate gets smaller but the degree of the improvement of the bit error rate gradually decreases. Since increasing the number of iterations results in the increase of decoding time, the number of iterations cannot be unlimitedly increased especially in real-time communication system.
According to the conventional decoding method, when a code rate is 1/n, the required number of constituent decoders is nxe2x88x921 and the number of log-likelihood ratios obtained at a certain decoding time is 1.
Accordingly, the present invention is directed to a decoding method of turbo codes and a device for the same that substantially obviates one or more of the limitations and disadvantages of the related art.
An objective of the present invention is to provide a decoding method of turbo codes and a device for the same, for activating constituent decoders in parallel, optimally weighting the components of log-likelihood ratio obtained at each constituent decoder to form a weighted log-likelihood ratio, and combining the weighted log-likelihood ratios to obtain diversity gain, thereby reducing a bit error rate and decreasing the number of iterations.
Additional features and advantages of the invention will be set forth in the following description, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure as illustrated in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages, and in accordance with the purpose of the present invention as embodied and broadly described, in a turbo decoder for code rate 1/n including nxe2x88x921 constituent decoders, nxe2x88x922 interleavers and nxe2x88x922 deinterleavers, a turbo code decoding method comprises: a first decoding process starting from a first constituent decoder among the nxc3x971 constituent decoders to generate a first log-likelihood ratio; a second decoding process being activated in parallel with the first decoding process and starting from a second constituent decoder to generate a second log-likelihood ratio different from the first log-likelihood ratio; a (nxe2x88x921)th decoding process being activated in parallel with other decoding processes and starting from a (nxe2x88x921)th constituent decoder to generate a (nxe2x88x921)th log-likelihood ratio different from the log-likelihood ratios; a step of optimally weighting respective components forming each log-likelihood ratio generated by each constituent decoder at each decoding process; and a step of combining the weighted log-likelihood ratios and performing the hard decision to obtain diversity gain.
In another aspect, the present invention provides a device for decoding turbo codes of code rate 1/n including nxe2x88x921 constituent decoders, nxe2x88x922 interleavers, and nxe2x88x922 deinterleavers, comprising: nxe2x88x921 constituent decoders, each for receiving parts of sequences from a transmitting party and extrinsic information from a previous constituent decoding as a priori information and for providing a log-likelihood ratio and extrinsic information; nxe2x88x922 interleavers respectively coupled to fronts of each constituent decoder other than a first constituent decoder, each for receiving parts of the sequence from the transmitting party and extrinsic information and scrambling them according to a particular rule to provide outputs; nxe2x88x922 deinterleavers, respectively coupled to backs of each constituent decoder other than the first constituent decoder, for receiving the log-likelihood ratio and extrinsic information and recovering original order of sequences to provide outputs; and a combiner for optimally weighting the components of log-likelihood ratio generated by each decoding process and combining nxe2x88x921 weighted log-likelihood ratios after a predetermined number of iterations, and providing a combined result.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.